lucy24 - 4:32 am on Jul 15, 2012 (gmt 0)
fwiw, I've got Piwik and haven't seen this one. I don't think it's significant that they give one of your own pages as referer. It's a standard robot technique* to lull your server's suspicions: "They're already on the site so they must already have been vetted".
* Or hotlinker. It seems to be how pinterest breezes past the ordinary barriers.